Mine has same issue. Not sure why, but it won't power on sometimes. I found if I flip the lock-unlock switch back and forth a few times, then press the power button at random intervals, it finally will come on. I think it's an internal flaw on the 810. I heard some even put it in the freezer for a while to get it to finally boot up.

It appears dead, but apparently, it's not. I don't know what causes it to do this. It is very frustrating.